This invention relates to a method of controlling sectional profiles of plates such as steel plates to be rolled by means of two, four, five and six-high-mills in thick plate rolling, hot or cold plate rolling, and more particularly to a rolling mill including particularly constructed rolls for controlling sectional profiles of plates to be rolled.
As to configuration and quality of rolled products, it has been severely required to eliminate four defects, that is, (a) waved deformations resulting from waving phenomenon (problem in flatness), (b) crowns due to difference in thickness between edges and centers, (c) edge drops owing to metal flowing particularly occurring in edges and (d) local protrusions (high spots, edge build-ups, etc.).
In general, when a material is being rolled to reduce its thickness, sectional profiles of the material in width directions are determined by deformations of axes of work rolls, flattened deformations of the rolls and thermal crown and wear of the rolls caused in rolling. This is the reason why the control of sectional profiles of plate to be rolled is needed.
In order to uniformly control the above configuration and quality of rolled products, i.e. flatness and thickness profiles, various methods have been proposed such as roll bending method, rolling schedule changing method (Japanese Laid-open Patent Application No. 55-92,215), method of combination of six-high HC mill shifting method four-high work roll shifting method with roll bending method (Japanese Patent Application Publication No. 7,635/76) and method of combination of four-high work roll shifting method with working rolls having one tapered ground ends (Japanese Laid-open Patent Application No. 55-77,903).
In order to prevent the waving control the crown of the material and reduce the edge drops, there has been no effective method other than carefully carrying out the rolling operation from the cold rolling to the hot rolling. Although the roll bending method or apparatus has been mainly used and is effective to control the flatness of the material to a certain extent, it is hardly effective to control the crown or edge drop reduction. Moreover, the rolling schedule changing method is not effective to control the edge drop reduction, although it is effective to control the crown so as to make it constant.
In the six-high HC mill, intermediate rolls are shifted dependently upon widths of material to be rolled and the roll bending action is combined therewith. In this case, if the intermediate rolls are further shifted inwardly, excess surface pressure occurs on the surfaces of the rolls to cause spalling to an extent such that the further inward shifting of the intermediate rolls cannot be actually realized. Accordingly, the crown-controlling performance is decreased and not effective to reduce the edge drops. Moreover, the construction and reconstruction cost are expensive.
Work rolls having tapered ground ends, so-called "trapezoidal crown" rolls make it possible to control crowns and the control edge drop reduction. And such work rolls are effective to prevent the waving if a roll bending apparatus is combined, because it improves the controlling of the crowns and edge drop reduction. However, when widths of plates to be rolled change, the control effect correspondingly changes and local protrusions cannot be prevented.
Namely as the local protrusions such as high spots, edge built-ups and the like are due to extraordinary wear of work rolls which would occur at constant distances from edges of material in width directions, prevention of the local protrusions is difficult in rolling mills whose work rolls assume constant positions.
Particularly, as the edge built-ups are caused by the extraordinary wear occurring at edges of the material which contact the tapered ground ends of the work rolls and whose temperature is lower than that of its center, the edge built-up tend to occur when plates of the same width are continuously rolled. Accordingly, edge built-ups occur more considerably in rolling with trapezoidal crown rolls which are required to maintain widths of plates to be rolled at a substantially constant value, so that tapered ground ends of work rolls contact the material at substantially the same location of the material.
In using the trapezoidal crown rolls, the edge built-ups and edge drops tend to increase when the quality or hardness of the material to be rolled is changed.
In rolling by means of a rolling mill including work rolls having one tapered ground ends according to the four-high work roll shifting method, on the other hand, it is effective to control the crown and the edge drops. However, once the configuration of one tapered ground ends of work rolls has been determined, such a control is not necessarily satisfactory when the quality and thickness of the material to be rolled are changed. Particularly, the control of edge drop reduction is insufficient and required to be more improved.